# Why alignment feature work only when output\_multiple not set **URL:** \ **Category:** GNU Radio\ **Created:** [June 16, 2015, 3:00pm UTC](https://www.ruby-forum.com/t/why-alignment-feature-work-only-when-output-multiple-not-set/242331 "2015-06-16T15:00:19Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![musicdenotation](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ea666f/32.png) [@musicdenotation](https://www.ruby-forum.com/u/musicdenotation)\ **Post date:** [June 16, 2015, 3:00pm UTC](https://www.ruby-forum.com/t/why-alignment-feature-work-only-when-output-multiple-not-set/242331/1 "2015-06-16T15:00:19Z")
Hi, After go through the block\_executor.cc, I found alignment feature work only when output\_multiple not set, why them can not work at the same time? – Thanks Tiankun ---
**Author:** ![Tiankun\_Hu](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/46a35a/32.png) [@Tiankun\_Hu](https://www.ruby-forum.com/u/Tiankun_Hu)\ **Post date:** [June 18, 2015, 2:31pm UTC](https://www.ruby-forum.com/t/why-alignment-feature-work-only-when-output-multiple-not-set/242331/2 "2015-06-18T14:31:22Z")
Hi Tom, Thanks your reply, I have another question, in function “min\_available\_space” why buffer\_size/2 is best ? ??? 2015???06???16??? 21:31, Tom R. ???: ---
**Author:** ![Tiankun\_Hu](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/46a35a/32.png) [@Tiankun\_Hu](https://www.ruby-forum.com/u/Tiankun_Hu)\ **Post date:** [June 16, 2015, 3:33pm UTC](https://www.ruby-forum.com/t/why-alignment-feature-work-only-when-output-multiple-not-set/242331/3 "2015-06-16T15:33:37Z")
On Tue, Jun 16, 2015 at 8:57 AM, Tiankun Hu [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Hi, > After go through the block\_executor.cc, I found alignment feature work > only when output\_multiple not set, why them can not work at the same time? > > – > Thanks > Tiankun Because they are competing objectives. The alignment tries to keep buffers aligned and therefore the number of items will be based on a multiple of the alignment requirement. If you need an output multiple that’s different than that, which one should the scheduler choose? Note that the buffers always begin on a page and so are inherently aligned. If your output multiple is also a multiple of the alignment for the data type, then you’ll always be aligned. Tom ---
**Author:** ![Tiankun\_Hu](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/46a35a/32.png) [@Tiankun\_Hu](https://www.ruby-forum.com/u/Tiankun_Hu)\ **Post date:** [June 19, 2015, 4:14pm UTC](https://www.ruby-forum.com/t/why-alignment-feature-work-only-when-output-multiple-not-set/242331/4 "2015-06-19T16:14:19Z")
Hi Tiankun, Hi Tom, I vaguely remember me wondering about that line. It comes from the single threaded scheduler; back then, wondering, I came to the conclusion that for the STS that it’s probably been considered useful to half the the “buffer usage ripple” that could occur when blocks started to always use the full buffer. That sounds helpful in a scheduler where, per iteration, you either do something or you don’t, so a buffer without any space left to write to might be worse than the overhead that you get by only being allowed to use buffer/2. Basically, back then, I had another problem, and just shrugged. Now, for the thread-per-block scheduler, there’s no monolithic iteration over all blocks, so I guess that problem wouldn’t occur. I would have to try; however, it’s hard to find a single proper benchmark for this (if it doesn’t break anything). Tiankun, maybe you have an idea? Best regards, Marcus ---
**Author:** ![Tiankun\_Hu](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/46a35a/32.png) [@Tiankun\_Hu](https://www.ruby-forum.com/u/Tiankun_Hu)\ **Post date:** [June 19, 2015, 3:16pm UTC](https://www.ruby-forum.com/t/why-alignment-feature-work-only-when-output-multiple-not-set/242331/5 "2015-06-19T15:16:28Z")
On Thu, Jun 18, 2015 at 8:29 AM, Tiankun Hu [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Hi Tom, > Thanks your reply, I have another question, in function > “min\_available\_space” why buffer\_size/2 is best ? I’m not really sure. That’s a question for Eric. What happens when you change it? How does it affect performance? Tom ---
**Author:** ![Tiankun\_Hu](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/46a35a/32.png) [@Tiankun\_Hu](https://www.ruby-forum.com/u/Tiankun_Hu)\ **Post date:** [June 19, 2015, 4:50pm UTC](https://www.ruby-forum.com/t/why-alignment-feature-work-only-when-output-multiple-not-set/242331/6 "2015-06-19T16:50:33Z")
On Fri, Jun 19, 2015 at 6:09 AM, Tom R. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > On Thu, Jun 18, 2015 at 8:29 AM, Tiankun Hu [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > wrote: > > > Hi Tom, > > Thanks your reply, I have another question, in function > > “min\_available\_space” why buffer\_size/2 is best ? > > I’m not really sure. That’s a question for Eric. The scheduler aims to keep the buffer no more than half full, so that in the steady state the producing block can write into the free half and the downstream consuming block(s) read from the filled half. ---
**Author:** ![Tiankun\_Hu](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/46a35a/32.png) [@Tiankun\_Hu](https://www.ruby-forum.com/u/Tiankun_Hu)\ **Post date:** [June 20, 2015, 5:06pm UTC](https://www.ruby-forum.com/t/why-alignment-feature-work-only-when-output-multiple-not-set/242331/7 "2015-06-20T17:06:32Z")
> But I don’t know why do it can increase parallelism? Assume the following situation file\_source-\> multiply\_const → file\_sink assume the complete file fits into the buffer, and n\_best wouldn’t be buffer/2, then file\_source would read the whole file, and write it to the buffer. Only then, multiply\_const would have something to work. It would also consume the whole input buffer at once, and produce the full output buffer. Only then, file\_sink could start to work. so, at no time, more than one block would be active. You could that run on a single-core computer, and wouldn’t see any difference to a multi-core computer. Even if the file is larger than the buffer, then multiply\_const couldn’t do anything, because it must wait for the whole time that file\_sink needs to write away a complete buffer. If you restrict the maximum number of produced samples to half the output buffer size, the multipy\_const block would much more often be able to execute, whilst file\_sink is still writing away half of the buffer. Greetings, Marcus ---
**Author:** ![Tiankun\_Hu](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/46a35a/32.png) [@Tiankun\_Hu](https://www.ruby-forum.com/u/Tiankun_Hu)\ **Post date:** [June 20, 2015, 4:44pm UTC](https://www.ruby-forum.com/t/why-alignment-feature-work-only-when-output-multiple-not-set/242331/8 "2015-06-20T16:44:55Z")
Hi Tom, Johnathan, Thanks your reply. Hi Marcus, Seems your conclusion is make sense. I think it might relate with buffer size which was allocated in “flat\_flowgraph.cc / allocate\_buffer” ``` // *2 because we're now only filling them 1/2 way in order to // increase the available parallelism when using the TPB scheduler. // (We're double buffering, where we used to single buffer) int nitems = s_fixed_buffer_size * 2 / item_size ``` But I don’t know why do it can increase parallelism? ??? 2015???06???19??? 22:48, Johnathan C. ???: ---
**Author:** ![Tiankun\_Hu](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/46a35a/32.png) [@Tiankun\_Hu](https://www.ruby-forum.com/u/Tiankun_Hu)\ **Post date:** [July 7, 2015, 3:29pm UTC](https://www.ruby-forum.com/t/why-alignment-feature-work-only-when-output-multiple-not-set/242331/9 "2015-07-07T15:29:22Z")
Hi Marcus, Sorry for responding lately. I got it, thanks your reply ??? 2015???06???20??? 23:05, Marcus M??ller ???: